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[Solved]: Disproving well-quasi-order by providing an infinite anti-chain

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Problem Detail: 

I am currently studying the theory behind Well-Quasi-Orders. However I am having some issues in understanding how an infinite anti-chain can be produced to disprove the claim that a partial order $P$ is a w.q.o.

In particular I am wondering whether it is logically sound to present as proof not the infinite anti-chain but rather an algorithm to produce it.

More specifically, I've been trying to solve an exercise from the following lecture which asks whether the class of $P_{3}\text{-free}$ graphs is a w.q.o or not on the induced subgraph operation $\leq_{i}$

I've proved that said class contains unions of cliques and defined an ordering of this set as follows $P=\lbrace G_{0},G_{1},G_{2},.... \rbrace$ with $G_{i}$ representing all $P_{3}\text{-free}$ graphs on $i$ vertices (and graphs in $G_{i}$ being represented in a touple-like alpharithmetic form $(a_{1},....,a_{i})$ such that $a_{1} \leq a_{2} \leq.... \leq a_{i}$ to avoid repetitions and $\sum_{j=1}^{i} a_{j}=i$ to define an order. For example $(0,0,2,2)$ $\in G_{4}$ encodes the graph with two disconnected $P_{2}$ components.

I have defined an algorithm $B$ that produces an anti-chain that grows bigger and bigger at each step

The algorithm uses gadgets defined as $B_{a,c}$ which represent the graph with $c+1$ connected components, the first $c$ of which are $K_{1}$(i.e a sole vertice) and the last component representing $K_{a}$

The steps of my algorithm are the following:

  1. Define $L$ as the sequence

  2. Add $B_{a,0}$ to $L$ ($a$ any number $>$ 0)

  3. repeat $\infty$ times:

    4.Replace each item $B_{a,c} \in L$ by $B_{a^{'}=a+1 + 10^{6},c}$ and produce $L^{'}$ this way.

    5.Let $X=B_{a^{'},c}$ be the graph with the maximum $c$ amongst those in $L^{'}$

    6.Add to $L=L^{'}$ the graph $B_{a^{'}-2,c+1}$

Certainly, this algorithm does create an anti-chain( in each step I create a graph with more connected components yet less vertices). However I am not sure that such a technique is valid for disproving w.q.o on the infinite anti-thesis claim because of the fact that it works on a step by step basis.

Any help is appreciated. Thank you very much!

Asked By : jjohn

Answered By : Yuval Filmus

Your algorithm produces an infinite sequence of antichains whose size is unbounded. However, it doesn't produce a single antichain of infinite size.

It is perfectly fine to specify an infinite chain by giving an algorithm that enumerates its members, and even more complicated constructions are possible (e.g. the priority method). However, the end result of the construction should be a single infinite chain rather than a sequence of antichains of unbounded size.

I suggest taking a look at Higman's lemma.

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